Energy generation and storage has long been a subject of study and development. Of special importance is the storage of energy in a compact portable system that can be easily charged and discharged, such as rechargeable batteries and capacitors. Individual components of such systems have been individually investigated and optimized generally by seeking to achieve a maximum stored energy density. However, most commercially exploited systems yield far less than their theoretical energy density.
One such energy storage system utilizes activated carbon electrodes to store ions therein, which upon discharge release the ions to generate an electrical current. An example of an activated carbon electrode system is the electric double layer capacitor system described in U.S. Pat. No. 3,536,963 to Boos. The mechanism for energy storage is based on the formation of an electrical double layer at the interface between an activated carbon electrode and a supporting electrolyte under an applied electric field. These devices will accept and store significant amounts of energy at any available potential over which the device is stable, unlike batteries where a given threshold voltage must be exceeded. Optimization of this system is based on the optimization of the charge storage capacity of the activated carbon electrode used therein. It has been found that the capacity of such an electric double-layer capacitor can amount to several tens of farads per gram of activated carbon when the activated carbon has a surface area in excess of 1000 m.sup.2 /g.
Other forms of energy storage also have been separately developed, such as chemical energy storage, wherein the energy is stored and/or released through coupled chemical reactions involving the exchange of electrons. Polyoxometalates species are known to sustain reversible, multiple electron transfer. See, for example, "Heteropoly and Isopoly Oxometalates", M. J. Pope, Springer-Verlag, New York City, N.Y. 1983.
Kummer et al. disclosed chemically regenerable redox fuel cells wherein the oxidant is oxygen and the fuel is hydrogen in U.S. Pat. Nos. 4,396,687 and 4,407,902. The fuel cells utilized anolyte and catholyte solutions, the catholyte solution containing a heteropoly acid catalyst for the regeneration of the catholyte in the presence of oxygen, and the anolyte solution containing a heteropoly acid as the active component.
Kodama and Nakamura disclosed the use of a heteropoly acid complex, H.sub.3 PMo.sub.12 O.sub.40 as a proton conducting solid electrolyte in a hydrogen/oxygen fuel cell, "Applications of Solid Electrolytes", T. Takahashi and A. Kozawa, Editors, pp. 38-39, JEC Press, Cleveland, Ohio, 1980.
Tell investigated the heteropolyanion complex H.sub.3 PW.sub.12 O.sub.40 as a solid electrolyte in an electrochromic display comprising SnO.sub.2 /H.sub.3 PW.sub.12 O.sub.40 /graphite, "Electrochromism in Solid Phosphotungstinic Acid", Journal of the Electrochemical Society, Vol. 127, p. 2451, 1980.
Metal oxides, such as the polyoxometalate V.sub.6 O.sub.13 have been used as positive electrode materials of rechargeable lithium cells, Abraham and Brummen, "Lithium Batteries", J. P. Gabano, Ed.; Academic Press, Ch. 14; New York, 1983. These oxides typically have extended lattice structures which undergo insertion on reduction.
In view of the above remarks, it is apparent that what is lacking in the field of activated carbon electrodes is an electrode having increased charge storage capacity to enable the further miniaturization of devices utilizing such electrode and/or to enable greater charge storage capacities in such devices.
It is therefore one object of the present invention to provide a modified activated carbon electrode having a relatively greater charge storage capacity.
It is a further object of the present invention to provide energy storage devices utilizing a modified activated carbon electrode having a relatively greater charge storage capacity.
It is yet another object of the present invention to provide an electric double layer capacitor incorporating modified activated carbon electrodes.
These and other objects of the present invention will become apparent to those skilled in the art from the below description of the invention.